Most if not all electrodeionization (EDI) systems require extensive pre-treatment including softening and reverse osmosis (RO) to remove hardness and excessive dissolved solids. EDI system influent limits on hardness are typically from <0.5 mg/L to <2.0 mg/L as calcium carbonate in order to prevent scale formation in the concentrating chamber(s) and cathode chamber(s). EDI systems are also limited in terms of the maximum total dissolved solids (TDS) and dissolved carbon dioxide that can be present in the influent. If these influent limits are exceeded, the cell will be overloaded and fail to produce the product water quality that is expected. For example, the influent specifications for several commercial EDI systems are given in Table 1 (information from manufacturers' product brochures).
TABLE 1MAXIMUMFEED TEA* ORPRODUCTHARDNESSCONDUCTIVITYOMEXELL EDI 210<2mg/L as CaCO3<25TEAOMEXELL EDI 210U<0.5mg/L<8TEAE-CELL MK-3<1.0mg/L<25TEAIONPURE LX-X<1.0<40uS/cmSnow Pure XL-R<1.0<33uS/cm*TEA = Total Exchangeable Anions as CaCO3. Dissolved carbon dioxide gas becomes bicarbonate, an exchangeable anion, in EDI systems.
All of these manufacturers also specify that the feed water must be RO permeate or equivalent. The reason why these commercial EDI systems have limited capacity for dissolved solids is because they are designed for continuous flow and they are removing the dissolved ions with electricity at the same rate that the dissolved ions enter the system. They have extremely limited capacity of regenerated resin and essentially no usable stored capacity. Because residential water usage patterns are intermittent, ranging from 0 gpm to over 6 gpm at peak flows, a purification device sized to purify peak flows instantaneously would be quite large and uneconomical.
A further drawback of conventional water softening using ion exchange resins is that the hardness in the water is replaced with sodium ions from an NaCl regenerant. NaCl regeneration produces brine waste consisting mainly of NaCl, CaCl2, MgCl2 and the like. Increasing environmental restrictions on discharge of brine wastes and salt-brine regenerated softeners makes removal of hardness from potable water without the use of NaCl and resulting brine waste desirable.